The newspaper quoted current and former government officials familiar with the matter as saying the intruders were able to copy and siphon data related to design and electronics systems, making it potentially easier to defend against the plane.

The spies could not access the most sensitive material, which is kept on computers that are not connected to the Internet, the paper added.

Citing people briefed on the matter, it said the intruders entered through vulnerabilities in the networks of two or three of the contractors involved in building the fighter jet.

Lockheed Martin Corp is the lead contractor. Northrop Grumman Corp and BAE Systems PLC also have major roles in the project. Lockheed Martin and BAE declined comment and Northrop referred questions to Lockheed, the paper said.

The Journal said Pentagon officials declined to comment directly on the matter, but the paper said the Air Force had begun an investigation.

The identity of the attackers and the amount of damage to the project could not be established, the paper said.